An inter-hemispheric comparison of the tropical storm response to global warming

Model studies do not agree on future changes in tropical cyclone (TC) activity on regional scales. We aim to shed further light on the distribution, frequency, intensity, and seasonality of TCs that society can expect at the end of the twenty-first century in the Southern hemisphere (SH). Therefore, we investigate TC changes simulated by the atmospheric model ECHAM5 with T213 (~60 km) horizontal resolution. We identify TCs in present-day (20C; 1969–1990) and future (21C; 2069–2100) time slice simulations, using a tracking algorithm based on vorticity at 850 hPa. In contrast to the Northern hemisphere (NH), where tropical storm numbers reduce by 6 %, there is a more dramatic 22 % reduction in the SH, mainly in the South Indian Ocean. While an increase of static stability in 21C may partly explain the reduction in tropical storm numbers, stabilization cannot alone explain the larger SH drop. Large-scale circulation changes associated with a weakening of the Tropical Walker Circulation are hypothesized to cause the strong decrease of cyclones in the South Indian Ocean. In contrast the decrease found over the South Pacific appears to be partly related to increased vertical wind shear, which is possibly associated with an enhanced meridional sea surface temperature gradient. We find the main difference between the hemispheres in changes of the tropical cyclones of intermediate strength with an increase in the NH and a decrease in the SH. In both hemispheres the frequency of the strongest storms increases and the frequency of the weakest storms decreases, although the increase in SH intense storms is marginal.     

Change of tropical cyclone heat potential in response to global warming

Tropical cyclone heat potential (TCHP) in the ocean can affect tropical cyclone intensity and intensification. In this paper, TCHP change under global warming is presented based on 35 models from CMIP5 (Coupled Model Intercomparison Project, Phase 5). As the upper ocean warms up, the TCHP of the global ocean is projected to increase by 140.6% in the 21st century under the RCP4.5 (+4.5 W m^-2 Representative Concentration Pathway) scenario. The increase is particularly significant in the western Pacific, northwestern Indian and western tropical Atlantic oceans. The increase of TCHP results from the ocean temperature warming above the depth of the 26°C isotherm (D26), the deepening of D26, and the horizontal area expansion of SST above 26°C. Their contributions are 69.4%, 22.5% and 8.1%, respectively. Further, a suite of numerical experiments with an Ocean General Circulation Model (OGCM) is conducted to investigate the relative importance of wind stress and buoyancy forcing to the TCHP change under global warming. Results show that sea surface warming is the dominant forcing for the TCHP change, while wind stress and sea surface salinity change are secondary.     

全球变暖、AMO、IPO对全球陆地降水变化的相对贡献

本文探究了不同海表温度(SST)模态对6—8月和12月—次年2月全球陆地降水的趋势以及年代际变化的相对贡献。首先对热带地区陆地降水和SST进行SVD分析,得到影响陆地降水的趋势和年代际变化主要的海洋模态为：海洋中的全球变暖(Global Warming, GW)、大西洋多年代际振荡(Atlantic Multidecadal Oscillation, AMO)和太平洋多年代际振荡(Interdecadal Pacific Oscillation, IPO)。其次利用多元线性回归模型进一步定量评估了全球变暖、AMO和IPO对不同地区陆地降水的相对贡献大小。结果表明,全球变暖对陆地降水变化的贡献在冬夏季都是最大的,AMO在6—8月的贡献次之。IPO在12月—次年2月的贡献次之。不同纬度带,三者的贡献不同。GW的贡献在6—8月期间对10°N以北地区较大,南半球受GW的贡献相对较小,GW在12月—次年2月对40°N以北降水贡献异常显著;AMO主要在6—8月对10°～40°S和50°～60°S纬度带上的降水变化的贡献比较大;而IPO主要在12月—次年2月对北半球中纬度降水变化的贡献比较大。GW对...     

Sensitivity of global warming to the pattern of tropical ocean warming

The current generations of climate models are in substantial disagreement as to the projected patterns of sea surface temperatures (SSTs) in the Tropics over the next several decades. We show that the spatial patterns of tropical ocean temperature trends have a strong influence on global mean temperature and precipitation and on global mean radiative forcing. We identify the SST patterns with the greatest influence on the global mean climate and find very different, and often opposing, sensitivities to SST changes in the tropical Indian and West Pacific Oceans. Our work stresses the need to reduce climate model biases in these sensitive regions, as they not only affect the regional climates of the nearby densely populated continents, but also have a disproportionately large effect on the global climate.

---

Present-day constraint for tropical Pacific precipitation changes due to global warming in CMIP5 models

The sensitivity of the precipitation responses to greenhouse warming can depend on the present-day climate. In this study, a robust linkage between the present-day precipitation climatology and precipitation change owing to global warming is examined in inter-model space. A model with drier climatology in the present-day simulation tends to simulate an increase in climatological precipitation owing to global warming. Moreover, the horizontal gradient of the present-day precipitation climatology plays an important role in determining the precipitation changes. On the basis of these robust relationships, future precipitation changes are calibrated by removing the impact of the present-day precipitation bias in the climate models. To validate this result, the perfect model approach is adapted, which treats a particular model’s precipitation change as an observed change. The results suggest that the precipitation change pattern can be generally improved by applying the present statistical approach.     

Sensitivity of direct global warming potentials to key uncertainties

The concept of global warming potential was developed as a relative measure of the potential effects on climate of a greenhouse gas as compared to CO₂. In this paper a series of sensitivity studies examines several uncertainties in determination of Global Warming Potentials (GWPs). For example, the original evaluation of GWPs for the Intergovernmental Panel on Climate Change (IPCC, 1990) did not attempt to account for the possible sinks of carbon dioxide (CO₂) that could balance the carbon cycle and produce atmospheric concentrations of CO₂ that match observations. In this study, a balanced carbon cycle model is applied in calculation of the radiative forcing from CO₂. Use of the balanced model produces up to 21% enhancement of the GWPs for most trace gases compared with the IPCC (1990) values for time horizons up to 100 years, but a decreasing enhancement with longer time horizons. Uncertainty limits of the fertilization feedback parameter contribute a 20% range in GWP values. Another systematic uncertainty in GWPs is the assumption of an equilibrium atmosphere (one in which the concentration of trace gases remains constant) versus a disequilibrium atmosphere (one in which the concentration of trace gases varies with time). The latter gives GWPs that are 19 to 32% greater than the former for a 100 year time horizons, depending upon the carbon dioxide emission scenario chosen. Five scenarios are employed: constant-concentration, constant-emission past 1990 and the three IPCC (1992) emission scenarios. For the analysis of uncertainties in atmospheric lifetime (τ) the GWP changes in direct proportion toτ for short-lived gases, but to a lesser extent for gases withτ greater than the time horizontal for the GWP calculation.     

Blunt ocean dynamical thermostat in response of tropical eastern Pacific SST to global warming

Using an intermediate ocean–atmosphere coupled model (ICM) for the tropical Pacific, we investigated the role of the ocean dynamical thermostat (ODT) in regulating the tropical eastern Pacific sea surface temperature (SST) under global warming conditions. The external, uniformly distributed surface heating results in the cooling of the tropical eastern Pacific “cold tongue,” and the amplitude of the cooling increases as more heat is added but not simply linearly. Furthermore, an upper bound for the influence of the equatorially symmetric surface heating on the cold tongue cooling exists. The additional heating beyond the upper bound does not cool the cold tongue in a systematic manner. The heat budget analysis suggests that the zonal advection is the primary factor that contributes to such nonlinear SST response. The radiative heating due to the greenhouse effect (hereafter, RHG) that is obtained from the multi-model ensemble of the Climate Model Intercomparison Project Phase III (CMIP3) was externally given to ICM. The RHG obtained from the twentieth century simulation intensified the cold tongue cooling and the subtropical warming, which were further intensified by the RHG from the doubled CO₂ concentration simulation. However, the cold tongue cooling was significantly reduced and the negative SST response region was shrunken toward the equator by the RHG from the quadrupled CO₂ concentration simulation, while the subtropical warming increased further. A systematic RHG forced experiment having the same spatial pattern of RHG from doubled CO₂ concentration simulation with different amplitude of forcing revealed that the ocean dynamical response to global warming tended to enhance the cooling in the tropical eastern Pacific by virtue of meridional advection and upwelling; however, these cooling effects could not fully compensate a given RHG warming as the external forcing becomes larger. Moreover, the feedback by the zonal thermal advection actually exerted the warming over the equatorial region. Therefore, as the global warming is intensified, the cooling over the eastern tropical Pacific by ODT and the negative SST response area are reduced.     

Assessment of uncertainties in the response of the African monsoon precipitation to land use change simulated by a regional model

Land use and land cover (LULC) over Africa have changed substantially over the last 60 years and this change has been proposed to affect monsoon circulation and precipitation. This study examines the uncertainties of model simulated response in the African monsoon system and Sahel precipitation due to LULC change using a set of regional model simulations with different combinations of land surface and cumulus parameterization schemes. Although the magnitude of the response covers a broad range of values, most of the simulations show a decline in Sahel precipitation due to the expansion of pasture and croplands at the expense of trees and shrubs and an increase in surface air temperature. The relationship between the model responses to LULC change and the climatologists of the control simulations is also examined. Simulations that are climatologically too dry or too wet compared to observations and reanalyses have weak response to land use change because they are in moisture or energy limited regimes respectively. The ones that lie in between have stronger response to the LULC changes, showing a more significant role in land–atmosphere interactions. Much of the change in precipitation is related to changes in circulation, particularly to the response of the intensity and latitudinal position of the African Easterly Jet, which varies with the changes in meridional surface temperature gradients. The study highlights the need for measurements of the surface fluxes across the meridional cross-section of the Sahel to evaluate models and thereby allowing human impacts such as land use change on the monsoon to be projected more realistically.     

Changes in aridity in response to the global warming hiatus

The global warming slowdown or warming hiatus, began around the year 2000 and has persisted for nearly 15 years. Most studies have focused on the interpretation of the hiatus in temperature. In this study, changes in a global aridity index (AI) were analyzed by using a newly developed dynamical adjustment method that can successfully identify and separate dynamically induced and radiatively forced aridity changes in the raw data. The AI and Palmer Drought Severity Index produced a wetting zone over the mid-to-high latitudes of the Northern Hemisphere in recent decades. The dynamical adjustment analysis suggested that this wetting zone occurred in response to the global warming hiatus. The dynamically induced AI (DAI) played a major role in the AI changes during the hiatus period, and its relationships with the North Atlantic Oscillation (NAO), Pacific Decadal Oscillation (PDO), and Atlantic Multi-decadal Oscillation (AMO) also indicated that different phases of the NAO, PDO, and AMO contributed to different performances of the DAI over the Northern Hemisphere. Although the aridity wetting over the mid-to-high latitudes may relieve long-term drying in certain regions, the hiatus is temporary, and so is the relief. Accelerated global warming will return when the NAO, PDO, and AMO revert to their opposite phases in the future, and the wetting zone is likely to disappear.     

全球变暖的有序适应问题

1 引言\n近百年全球变暖已是不争的事实。 其原因有不同说法, 但绝大多数科学家认为和人类活动有关。 例如, 建立在各国研究成果基础上的政府间气候工作组评估报告认为, 近代全球变暖主要是人类活动排放大量二氧化碳等大气温室气体所造成的（IPCC,2007）。观测证据显示,目前人类活动排放及大气温室气体的增长趋势仍在持续。由于大气中二氧化碳具有较长存在寿命以及海洋系统的热惯性等原因,即使现在就停止各种排放活动, 导致全球变暖的强迫力仍将在未来相当长时期内持续下去。因此,全球变暖作为一个事实已成为越来越多决策者必须考虑的问题。 近年来频繁地出现在G8和G20等国家[CM(22]首脑会议的议程中的一个议题就是： 应对全球[CM)]变暖。\n由于涉及各国经济发展利益,如何应对全球变暖已不是一个单纯的科学问题。目前很多国家\n已开始重视减排以及清洁能源的发展和利用问题,因而有了联合国主持的巴厘岛会议。这是象征各国政府共同意识到必须协调全球人类活动的一次重要会议。然而,即使仅从科学研究角度提出如何适应全球变暖问题,目前科学界却并无明确的解决思路。\n　　工业革命以来,人类在经济利益驱动下的大规模而无序的生产活动,导致大量大气温室气体排放促成当前的全球变暖。这可被视为人类历史上一次大规模无序活动影响全球气候系统的例证（叶笃正等,2001）。全球变暖已对各地产生了重大影响,就如何适应全球变暖而言,现在我们面临潜在的又一波无序的人类活动,即各国以维护本国利益为主导来展开适应全球变暖的行动。显然,各国都会尽量利用其正面影响,想方设法减轻或消除其负面影响。直接后果是导致各国当地经济和气候环境变化。然而,没有一个国家是封闭的,它的经济发展必然和其他地区有关。气候环境更是如此,一地的气候变化会影响到其他地区。因而,各国各行其是的后果是否导致人类整体利益的更大损害,不得而知。为避免新的一波人类无序活动所可能产生的不利影响,开展相应的科学研究,并在此基础上协调各国以形成全球有序适应,是有益的。\n　　在此,我们提出构建全球有序适应的一个思路。\n　　首先,需要开展更为全面的地球系统科学模拟研究,把人类活动作为地球系统中一个相互作用的组成部分,研究全球气候环境的演变（叶笃正,季劲钧,1998;叶笃正等,2009）。目前很多国家及一些国际科学计划已在开展关于全球变暖对各地经济发展的影响和适应研究,部分地反映在IPCC系列报告中。进而有必要在此研究基础上,发展模型模拟各种虚拟的适应方案,和由此导致的各种进一步的气候环境变化,及其对于各地社会和经济的反馈影响。这个虚拟试验研究应该由有关国际组织（如世界气象组织WMO）来组织实施,通过比较分析各种试验结果,总结出最有利于全球各地整体利益的几个最佳适应方案。这就是理想的全球有序适应方案。\n对于某些国家来说, 全球有序适应方案也可能会导致某些经济利益的损失。诚如此言,就涉及科学层面以外的国家利益问题了。 因此, 全球有序适应的实施必须通过有关国际机构（如联合国）来调控。 如果某些国家因参与全球有序适应而受到利益损失, 可以由其他国家（通过联合国某种基金的形式）来给予补偿。 目前国际社会在应对全球变暖问题方面尚缺乏整体措施, 亟待各国科学家以及其他[CM(22]有关力量协作,在国际层面达成共识并尽快付诸行动。\n　　需要指出,全球有序适应（adaptation）并不排斥减排措施（mitigation）。在执行各种国际减排公约的同时,各国应有序适应全球变暖。即要强调全球整体的发展和可持续性,达到整体利益的最佳,而非某个国家或地区局部的最佳。     

The response of warm-season precipitation extremes in China to global warming: an observational perspective from radiosonde measurements

Consensus has been reached that precipitation extremes vary proportionally with global warming. Nevertheless, the underlying cause and magnitude of these factors affecting their relationships remain highly debated. To elucidate the complex relationship between precipitation extremes and temperature in China during the warm seasons (May through September), a 60-year (1958–2017) record of hourly rain gauge measurements, in combination with surface air temperature, RH, precipitable water (PW), and convective available potential energy (CAPE) collected from 120 radiosonde stations were examined. Spatially, the scaling relationship between precipitation extremes and temperature exhibits a large geographic difference across China. In particular, the Clausius–Clapeyron (CC) and sub-CC relationships tend to occur in northwest (ROI-N) and southeast China (ROI-S), whereas the super-CC relationship is found to mainly concentrates in central China (ROI-C). Additionally, the response of precipitation extremes to temperature becomes more sensitive as precipitation intensity increases, shifting from CC to super-CC at a certain point of inflection that varies by geographic regions. This shift occurs at approximately 15 °C in ROI-C and ROI-N, but at around 20 °C in ROI-S. Within the temperature range of the super-CC slope, the PW rises with the increases in temperature, whereas the CAPE decreases with rising temperature, which is contrary to the monotonic scaling of precipitation with temperature. From the perspective of interannual variation, the precipitation extremes correlate positively with temperature. This further confirms the notion that global warming, through jointly affecting PW and CAPE, is able to considerably regulate precipitation extremes.     

Increased population exposure to precipitation extremes in China under global warming scenarios

极端降水(暴雨)是我国最为主要的自然灾害之一,每年均造成巨大经济损失和人员伤亡。现有预估研究表明,未来全球持续增暖使得我国极端降水发生频次显著增加,强度增强。那么,未来极端降水增加会对社会,尤其是人们生活造成多大影响?围绕这个问题,本研究基于多个高分辨率区域气候模式模拟和人口数据,分析了未来我国极端降水人口暴露度的可能变化。结果指出,在RCP4.5-SSP2情景下,到了21世纪末,虽然预估的我国人口数量大幅减少,但极端降水人口暴露度却显著增加,相对当前气候增加了约21.6%,其中东部地区是增加最为显著的区域。进一步研究发现,人口暴露度的增加不依赖于情景的选择,但高排放情景增加幅度更大,而且增加主要是由于气候变化的贡献。     

Tropical land savannization: impact of global warming

This study investigates the impact of global warming on the savannization of the tropical land region and also examines the relative roles of the impact of the increase of greenhouse gas concentration and future changes in land cover on the tropical climate. For this purpose, a mechanistic–statistical–dynamical climate model with a bidirectional interaction between vegetation and climate is used. The results showed that climate change due to deforestation is more than that due to greenhouse gases in the tropical region. The warming due to deforestation corresponds to around 60% of the warming in the tropical region when the increase of CO₂ concentration is included together. However, the global warming due to deforestation is negligible. On the other hand, with the increase of CO₂ concentration projected for 2100, there is a lower decrease of evapotranspiration, precipitation and net surface radiation in the tropical region compared with the case with only deforestation. Differently from the case with only deforestation, the effect of the changes in the net surface radiation overcomes that due to the evapotranspiration, so that the warming in the tropical land region is increased. The impact of the increase of CO₂ concentration on a deforestation scenario is to increase the reduction of the areas covered by tropical forest (and a corresponding increase in the areas covered by savanna) which may reach 7.5% in future compared with the present climate. Compared with the case with only deforestation, drying may increase by 66.7%. This corroborates with the hypothesis that the process of savannization of the tropical forest can be accelerated in future due to global warming.     

The ratio of land to ocean temperature change under global warming

The result in climate simulations, supported in the observation-based record, is that the ratio $\phi = T_{L} /T_{O} $ of land-average to ocean-average temperature change is greater than one and varies comparatively modestly as climate changes. This is investigated in results from the CMIP3 data archive of climate change simulations following the B1 and more strongly forced A1B scenarios as well as in 2×CO₂ integrations. The associated precipitation ratio $ \psi = P_{L} /P_{O} $ is also considered briefly. The behaviour of ? is analyzed in terms of a forcing-response view of the energy balance over land and ocean regions. The analysis indicates that the value of ??>?1 is not maintained by separate local balances over land and ocean but by an energetic balance that also involves a change in transport between the regions. The transport change does not restrain the land warming by exporting energy to the ocean region but, rather, the reverse. The anomalous transport is from the ocean to the land region even though the ocean warms less than the land does. Feedbacks in the ocean region, especially in the equatorial Pacific, do not sufficiently counteract the forcing and the result is an excess of energy that is transported to the land. The land warms in order to radiate away both the energy from the forcing over land but also the extra energy imported from the ocean region, thereby maintaining ??>?1. This situation can be understood to parallel the SST-forced case in model studies where ??>?1 despite the forcing being confined to the ocean area. The climate system is effective in redistributing forcing so that it is the local feedbacks, rather than the pattern of the forcing, that determine the temperature response. Land and ocean averaged quantities and budgets behave in a consistent manner to provide a simplified representation of the changes in temperature and energetic processes that are occurring. The geographical distributions of the terms do not, however, display a strong land/ocean demarcation. The land/ocean average budgets and balances are the residual of processes that vary considerably within the land and ocean boundaries.     

Eddy parametrization and the oceanic response to idealized global warming

 A coarse-grid global ocean general circulation model (OGCM) is used to determine the role of sub-grid scale eddy parametrization schemes in the response to idealized changes in the surface heat flux, of the same order as expected under increased atmospheric CO₂ concentrations. Two schemes are employed. The first (H) incorporates standard horizontal mixing, whereas the second (G) combines both enhanced isopycnal mixing and eddy-induced transport. Uniform surface heating anomalies of +2 W m^-2 and −2 W m^-2 are applied for 50 years, and the results are compared with a control experiment in which no anomalous heating is imposed. A passive “heat” tracer is applied uniformly (at a rate of 2 W m^-2 for 50 years) in a separate experiment. The sea-surface temperature response to global surface heating is generally larger in G, especially in the northern subtropical gyres, along the southern coast of Australia and off the Antarctic coast. A pronounced interhemispheric asymmetry (primarily arising from an anomalous response south of 35 °S) is evident in both H and G. The surface trapping of passive tracers in the Southern Hemisphere is generally greater in G than it is in H, and is particularly pronounced along the prime meridian (0 °E). Dynamical changes (i.e., changes in horizontal and vertical currents, convection, and preferred mixing and eddy transport pathways) enhance surface warming in the tropics and subtropics in both G and H. They are dominated by an anomalous meridional overturning centred on the equator, which may also operate in greenhouse warming experiments using coupled atmosphere-ocean GCMs. Over the Southern Ocean the passive tracer experiments and associated ventilation rates suggest that surface warming will be greater in G than in H. In fact, the contrast between the dynamical responses evident in G and H in the actual heating experiments leads to a situation in which the reverse is often true. Overall, dynamical changes enhance the interhemispheric assymetry, more so in G than in H. Received: August 1996/Accepted: 20 March 1997     

Modelling the short-term response of the Greenland ice-sheet to global warming

 A two-dimensional vertically integrated ice flow model has been developed to test the importance of various processes and concepts used for the prediction of the contribution of the Greenland ice-sheet to sea-level rise over the next 350 y (short-term response). The mass balance is modelled by the degree-day method and the energy-balance method. The lithosphere is considered to respond isostatically to a point load and the time evolution of the bedrock follows from a viscous asthenosphere. According to the IPCC-IS92a scenario (with constant aerosols after 1990) the Greenland ice-sheet is likely to cause a global sea level rise of 10.4 cm by 2100 AD. It is shown, however, that the result is sensitive to precise model formulations and that simplifications as used in the sea-level projection in the IPCC-96 report yield less accurate results. Our model results indicate that, on a time scale of a hundred years, including the dynamic response of the ice-sheet yields more mass loss than the fixed response in which changes in geometry are not incorporated. It appears to be important to consider sliding, as well as the fact that climate sensitivity increases for larger perturbations. Variations in predicted sea-level change on a time scale of hundred years depend mostly on the initial state of the ice-sheet. On a time scale of a few hundred years, however, the variability in the predicted melt is dominated by the variability in the climate scenarios. Received: 21 August 1996/Accepted: 12 May 1997